1. Field
The present disclosure relates to light emitting diodes (LEDs) and methods of fabricating the same.
2. Discussion of the Background
In general, Group-III-element nitrides, such as gallium nitride (GaN) and aluminum nitride (AlN), have excellent thermal stability and a direct-transition-type energy band structure. As such, Group-III-element nitrides have recently come into the spotlight as materials for light emitting devices that emit visible and ultraviolet wavelengths. Particularly, blue light emitting devices and green light emitting devices using indium gallium nitride (InGaN) are used in various applications, such as large-sized full-color flat panel displays, traffic lights, indoor illumination, high-density light sources, high-resolution output systems, and optical communications.
Since it is difficult to fabricate a homogeneous substrate for the growth of such Group-III-element nitride semiconductors, Group-III-element nitride semiconductor layers are generally grown on a heterogeneous substrate having a similar crystal structure to that of the semiconductor layer, through processes such as metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). A sapphire substrate having a hexagonal system structure is frequently used as the heterogeneous substrate. However, since sapphire is an electrical insulator, it limits the structure of a light emitting diode (LED) grown thereon. Accordingly, a technique has recently been developed in which epitaxial layers, such as nitride semiconductor layers, are grown on a heterogeneous substrate such as sapphire. A support substrate is bonded to the epitaxial layers, and the heterogeneous substrate is then separated using a laser lift-off technique or the like, thereby fabricating a high-efficiency vertical-type LED (e.g., see U.S. Pat. Nos. 6,744,071 and 7,704,763).
Such a vertical-type LED is fabricated by sequentially forming an n-type GaN layer, an active layer, and a p-type GaN layer on a sapphire growth substrate, forming a p-electrode and a reflective metal layer on the p-type GaN layer, bonding a support substrate thereon, removing the sapphire substrate, and then forming an n-electrode or n-electrode pad on the exposed n-type semiconductor layer. A conductive substrate is generally used as the support substrate. Thus, the LED has a vertical structure, in which the n-electrode and the p-electrode are disposed opposite to each other.
However, Ag, which is frequently used as a reflection layer while being in ohmic-contact with the p-type GaN layer, may become aggregated during a heating process. Further, since current leakage may occur due to the migration of Ag atoms when the LED is driven, it is difficult to form a stable ohmic metal reflection layer. Furthermore, Ag also has reflectance limitations.
FIG. 1 is a sectional view illustrating a conventional vertical-type LED. Referring to FIG. 1, the conventional LED is fabricated by sequentially forming a GaN-based n-type layer 1, a GaN-based active layer 2, and a GaN-based p-type layer 3 on a growth substrate (not shown), to form a light emitting structure 4. Then, a p-type electrode 5 is formed on the p-type layer 3, the p-type electrode 5 is flip-bonded to a Si submount 7 through a bonding metal 6, the growth substrate is removed, and then an n-electrode 8 is formed on the exposed n-type layer 1. Meanwhile, an n-type electrode 9 is formed on the bottom surface of the Si submount 7. Furthermore, in U.S. Pat. No. 7,704,763, the surface of the exposed n-type layer 1 is roughened using a dry or photo-electro chemical (PEC) etching technique, thereby enhancing the light extraction efficiency thereof.